late 13c., from corner (n.) + stone (n.). The figurative use is from early 14c.

I endorse without reserve the much abused sentiment of Governor M'Duffie, that "Slavery is the corner-stone of our republican edifice;" while I repudiate, as ridiculously absurd, that much lauded but nowhere accredited dogma of Mr. Jefferson, that "all men are born equal." No society has ever yet existed, and I have already incidentally quoted the highest authority to show that none ever will exist, without a natural variety of classes. [James H. Hammond, "Letter to an English Abolitionist" 1845]